FIGS. 9 to 11 exemplify a typical making process of a pneumatic radial tire. FIG. 9 shows a band making step in which a sheet-shaped carcass layer arranging numerous steel cords in parallel is wrapped on a making drum 90 to form a cylindrical band 91. Subsequently, the cylindrical band 91 is detached from the making drum 90 and moved onto a making drum 93 for a first green tire making step as shown in FIG. 10. On the making drum 93, bead cores 92 and 92 are respectively inserted from outside to both end portions of the cylindrical band 91, and then the both end portions of the cylindrical band 91 are folded back as indicated by arrows so as to wrap these bead cores 92 and 92. A first green tire is made by further wrapping side treads on the outer peripheries of the both end portions thus folded back. Subsequently, the first green tire is detached from the making drum 93 and moved onto bead rings 98 for a second green tire making step as shown in FIG. 11. On the bead rings 98, both end portions in the width direction of the first green tire 94 are pulled toward the center and a central portion is expanded at the same time. The first green tire 94 is shaped into a toroidal green tire 97 by wrapping a belt layer 95 and a cap tread layer 96 on the periphery of the expanded central portion. Lastly, the second green tire 97 is taken off from the bead rings 98 and put into a mold, and is subjected to vulcanization and curing in the state pressed against an inner surface of the mold while expanding a bladder from inside to form a product tire.
In the meantime, a pneumatic tire for a construction vehicle is characterized by an extremely heavy weight in comparison with passenger car tires and normal truck and bus tires. For example, a relatively small tire in the tire size of the 1800R33 class has a weight equal to or above 450 kg, while a tire in the medium tire size of the 2700R49 class exceeds 1400 kg. Since the pneumatic radial tire for a construction vehicle is enormously heavy as described above, the green tire may be deformed in the aforementioned tire making step not only due to an external force from outside but also due to its own weight, and may be formed into a tire having low uniformity when subjected to vulcanization while containing deformation. Conventionally, to prevent such deformation, an especially major-scale shape retaining auxiliary apparatus has been used in the respective making steps for the pneumatic tire for a construction vehicle.
As a result of investigation by the inventors of the present invention, it was found out that a lot of deformation in the making steps for the above-described enormously heavy green tire mainly occurred in junctions on the carcass layer, and that tension applied in the circumferential direction of the tire relative to carcass cords was a major cause thereof. For example, in the band making step, locally uneven widening (open cords) occurs between the carcass cords pivotally around the junctions on the carcass layer when detaching the cylindrical band from the making drum. Meanwhile, the tension in the circumferential direction may occur locally when suspending the cylindrical drum at the time of moving the cylindrical band from the making drum of the band making step to the making drum of the first green tire making step or when expanding the first making drum after inserting the cylindrical band into the first making drum, whereby locally uneven widening or disorder in the arrangement of the cords is apt to occur between the carcass cords.
Moreover, in the second green tire making step, uneven widening is apt to occur between the cords when expanding the first green tire into a toroidal shape. Meanwhile, in the step of setting the second tire into the mold and performing vulcanization, the weight is further increased by assembling the belt layer and the tread rubber. Accordingly, the second green tire is subject to deformation due to its own weight.
As described previously, prevention of deformation in the making steps for the pneumatic radial tire for a construction vehicle is carried out by use of the major-scale shape retaining auxiliary apparatus. However, the major-scale apparatus requires not only excessive plant costs but also a space for installation and more operating procedures, thereby leading to a decline in productivity. Accordingly, contribution to cost reduction will be immense if it is possible to achieve prevention of deformation without using the shape retaining auxiliary apparatus. However, a solution thereto is not yet to be found.
In terms of a heavy-duty radial tire for a normal truck or bus, which is lighter than the tire for a construction vehicle, there is one configured to insert a reinforcing layer made of organic fiber cords between a carcass layer and an inner liner in a tread shoulder region (see Patent Document 1). However, it is impossible to obtain an effect to prevent uneven widening between the cords on the carcass layer merely by applying such a reinforcing layer simply to the pneumatic radial tire for a construction vehicle of the enormously heavy structure.
Patent Document 1: Japanese patent application Kokai publication No. 5-319015